1. Field of the Invention
The present invention relates to a backlight unit in use for an LCD panel, and more particularly, to an LED driving circuit of a backlight unit which has an improvement in driving current detection, by which driving current of a light source of the backlight unit such as LEDs can be controlled at a constant current and thus be stabilized.
2. Description of the Related Art
In general, a backlight source for a Liquid Crystal Display (LCD) panel such as an LCD TV and LCD monitor uses a light-emitting device such as a Cold Cathode Fluorescent Lamp (CCFL) and a Light Emitting Diode (LED).
The CCFL has some drawbacks such as environmental contamination due to mercury gas contained therein, low response rate of about 15 ms, low color reproducibility and inaptness to a light-weighted and small-sized LCD panel.
To the contrary, the LED has various merits such as environmental friendless, rapid response rate of several nano seconds, which enables high speed response and makes the LED suitable to video signal stream processing, capability of impulsive action, high color reproducibility of substantially 100%, change in brightness and color temperature through the adjustment of light quantity of red, green and blue LEDs and aptness to a light-weighted and small-sized LCD panel. At present, LEDs with such merits are actively adopted as a backlight source of an LCD panel and so on.
In a backlight unit in use for such an LCD panel, a light source includes a plurality of LEDs connected in series, where a forward voltage offset exists in the LEDs. So, a constant voltage driver or driving circuit is necessary to drive the LEDs connected in series at a constant current.
Such a driving circuit for driving the LEDs that is a light source of a backlight unit for an LCD panel may be constructed of a buck type or boost type DC-DC converter.
Such an LED driving circuit of a backlight unit in use for an LCD panel adopting a PWM boost type DC-DC converter is illustrated in FIG. 1.
In the conventional LED driving circuit of a backlight unit as shown in FIG. 1, an anode of a diode D is connected through an inductor L to an input of direct voltage Vin, and a smoothing capacitor C and an LED array 11 including a plurality of LEDs are connected in parallel between a cathode of the diode D and a ground. A switch SW is connected between the ground and a node N that connects the inductor L with the diode D. The switch SW is constructed of a Metal Oxide Semiconductor (MOS) transistor, and a voltage-detecting resistor Rs is connected between the source of the MOS transistor and the ground.
A voltage Vs detected by the voltage detecting resistor Rs is inputted to a Pulse Width Modulation (PWM) controller 10, which in turn controls on/off duty ratio of a switching pulse according to the magnitude of the detection voltage Vs and outputs the controlled voltage to the gate of the MOS transistor, that is, the switch SW. Such PWM control using a switching pulse will be described below with reference to FIG. 2.
FIG. 2 illustrates a PWM control timing chart of the conventional LED driving circuit of a backlight unit in use for an LCD panel.
Referring to (a) to (c) of FIG. 2, when the detection voltage Vs is substantially the same as internal reference voltage, the PWM controller 10 outputs a switching pulse SP with a duty ratio of approximately 50%, as shown in FIG. 2(a), to the switch SW. If the detection voltage Vs is larger than the internal reference voltage, a switching pulse SP with a duty ratio over 50%, as shown in FIG. 2(b), is outputted to the switch SW. Furthermore, if the detection voltage Vs is smaller than the internal reference voltage, a switching pulse SP with a duty ratio under 50%, as shown in FIG. 2(c), is outputted to the switch SW.
According to such switching pulses SP, when the switch SW is on, current originating from the direct voltage Vin flows through the inductor L and the switch SW, in which energy is stored in the inductor L. When the switch SW is off, the sum of energy of the direct voltage Vin and that stored in the inductor L is transferred to the LED array 11 through the diode D. Here, the voltage transferred to the LED array 11 is smoothed by the smoothing capacitor C, and its value is the same as or larger than input voltage.
Such PWM control is carried out based on the detection voltage Vs, which is detected at the voltage detecting resistor Rs, which in turn detects voltage from driving current supplied to the LED array 11. Such driving current detected by the voltage detecting resistor Rs will be described below with reference to FIG. 3.
FIG. 3 illustrates a driving current detection timing chart of the conventional LED driving circuit of a backlight unit in use for an LCD panel.
As shown in FIG. 3, when the switch SW is on, current originating from the direct voltage Vin is detected at the switch SW when flows through it. However, when the switch SW is off, current is not detected. When the switch SW is on, as time goes on, energy stored in the inductor and the capacitor makes increasingly more current flow through the switch. As shown in FIG. 3, such current shows a triangular waveform.
However, such driving current detection by the conventional LED driving circuit of a backlight unit in use for an LCD panel has following drawbacks. That is, in the switch-on status, current rises with a specific slope and then drops sharply, and in the switch-off status, current does not flow through the voltage detecting resistor Rs. As a result, current detection intervals alternate with no-detection intervals, leaving a series of detection blanks. Then, precise current detection can be carried out hardly and thus output current can be rarely maintained at a constant value.